Endothermic cracking aircraft fuel system

ABSTRACT

A method of controlling cooling in an aircraft system includes endothermically cracking a fuel to increase its cooling capacity using a catalyst that includes at least one transition metal compound of at least one of carbides, nitrides, oxynitrides, oxycarbonitrides, oxycarbides, phosphides, and combinations, and the transition metal includes at least one of zirconium, hafnium, tantalum, niobium, molybdenum, tungsten, platinum, palladium, rhodium, iridium, ruthenium, osmium, rhenium, and combinations thereof. The cracked fuel is used to cool a heat source that includes an aircraft component.

REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of U.S. patent application Ser. No.13/198,741, filed Aug. 5, 2011, which is a Divisional of U.S. patentapplication Ser. No. 11/507,382, filed on Aug. 21, 2006, now U.S. Pat.No. 8,015,823.

BACKGROUND OF THE INVENTION

This disclosure relates to aircraft fuel systems and, more particularly,to increasing a cooling capacity of fuel within the fuel system to coolheat-producing aircraft components.

Fuel is widely known and used in the aircraft industry as a heat sinkbefore combustion for cooling heat-producing aircraft components. Forexample, in aircraft having gas turbine engines, the fuel is used tocool bleed air from an engine compressor in a cabin air cycle controlsystem, heat-producing aircraft components in a thermal managementsystem, or an engine turbine in a turbine film cooling system.

To extend the usefulness of the fuel as a coolant, there have beenproposals to treat the fuel to increase the cooling capacity. Forexample, the cooling capacity of dissolved oxygen-rich fuel is limitedbecause the oxygen initiates formation of deposits commonly referred toas “coke” or “coking” at temperatures between 350° F. and 850° F.Typically, lowering the oxygen concentration overcomes the cokingproblem and allows the fuel to be heated without significant coking.Thus, an oxygen-depleted or “deoxygenated” fuel can be heated to ahigher temperature without coking to provide increased cooling capacity.

For some aircraft, an even greater cooling capacity is desired. Forexample, hypersonic aircraft or other types of propulsion devices,components or engines (e.g., scramjet engines) may operate attemperatures near or above 850° F. and up to about 1700° F. At suchtemperatures, the traditional fuel treatment may not provide fuel havingenough cooling capacity to cool the aircraft components or engine to adesired temperature without coking. Therefore, there is a need for afuel system and method for increasing the cooling capacity of the fuel.

SUMMARY OF THE INVENTION

A method of controlling cooling in an aircraft system according to anexample of the present disclosure includes endothermically cracking afuel to increase its cooling capacity using a catalyst that includes atleast one transition metal compound of at least one of carbides,nitrides, oxynitrides, oxycarbonitrides, oxycarbides, phosphides, andcombinations thereof. The transition metal includes at least one ofzirconium, hafnium, tantalum, niobium, molybdenum, tungsten, platinum,palladium, rhodium, iridium, ruthenium, osmium, rhenium, andcombinations thereof. The cracked fuel is used to cool a heat sourcecomprising an aircraft component.

In a further embodiment of any of the forgoing embodiments, theendothermic cracking includes cleaving carbon-to-carbon bonds of thefuel.

In a further embodiment of any of the forgoing embodiments, thecarbon-to-carbon bonds are of non-aromatic hydrocarbon chains of thefuel.

In a further embodiment of any of the forgoing embodiments, theendothermic cracking includes dehydrogenation of the fuel.

In a further embodiment of any of the forgoing embodiments, theendothermic cracking includes endothermically cracking a pre-selectedtype of a compositional constituent of the fuel to absorb a first amountof heat and to produce a product compositional constituent.

In a further embodiment of any of the forgoing embodiments, theendothermic cracking includes endothermically cracking the productcompositional constituent to absorb a second amount of heat.

In a further embodiment of any of the forgoing embodiments, theendothermic cracking includes endothermically cracking the predeterminedtype of the compositional constituent using a first catalyst sectionhaving a first transition metal and endothermically cracking the productcompositional constituent using a second catalyst section having asecond, different transition metal.

A further embodiment of any of the foregoing embodiments includesregenerating the catalyst to increase catalytic activity.

In a further embodiment of any of the forgoing embodiments, theregenerating includes adding an oxygenate or a nitrogen-containingcompound to the fuel as a source of, respectively, oxygen or nitrogenfor regenerating the catalyst.

In a further embodiment of any of the forgoing embodiments, thetransition metal compound further includes a metal selected from thegroup consisting of aluminum, scandium, titanium, vanadium, chromium,manganese, silicon, thorium, and combinations thereof.

In a further embodiment of any of the forgoing embodiments, thetransition metal is selected from the group consisting of tantalum,niobium, molybdenum, tungsten, and combinations thereof.

In a further embodiment of any of the forgoing embodiments, the catalystincludes at least one of niobium oxynitride, niobium oxycarbide, andmolybdenum oxynitride.

In a further embodiment of any of the forgoing embodiments, theendothermic cracking includes endothermically cracking a predeterminedtype of a compositional constituent using a first catalyst sectionhaving at least one of niobium oxynitride or niobium oxycarbide toproduce a product compositional constituent, and endothermicallycracking the product compositional constituent using a second catalystsection having molybdenum oxynitride.

BRIEF DESCRIPTION OF THE DRAWINGS

The various features and advantages of this invention will becomeapparent to those skilled in the art from the following detaileddescription of the currently preferred embodiment. The drawings thataccompany the detailed description can be briefly described as follows.

FIG. 1 schematically illustrates an example aircraft fuel system havinga fuel passage that extends through an aircraft wing.

FIG. 2 schematically illustrates a second embodiment of an aircraft fuelsystem having a fuel passage within an aircraft engine.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 illustrates selected portions of an example aircraft fuel system20 for increasing a cooling capacity of an aircraft fuel. In thisexample, the aircraft fuel system 20 includes a storage portion 22 forstoring the aircraft fuel. The aircraft fuel is conveyed through a fuelpassage 24 to aircraft engine 26, such as a scramjet engine. A fuelstabilization unit 27, such as a fuel deoxygenator, associated with thefuel passage 24 removes oxygen from the fuel. In this example, the fuelpassage 24 extends within an aircraft wing 28 to cool the aircraft wing28.

The fuel passage 24 includes walls 30 that define the passage for theaircraft fuel. A catalyst 32 adjacent the walls 30 endothermicallycracks the aircraft fuel to increase the cooling capacity of the fuelfor absorbing heat from the aircraft wing 28. The term “crack” or“cracking” as used in this description refers to decomposing a moleculeor molecules into lighter molecules. The decomposition reaction absorbsheat to thereby increase the amount of heat that the aircraft fuelabsorbs from a heat source. In the illustrated example, air 34 travelingacross the aircraft wing 28 frictionally heats the aircraft wing 28. Theaircraft fuel passing through the fuel passage 24 absorbs the heat 36 tocontrol the temperature of the aircraft wing 28.

FIG. 2 illustrates selected portions of another example aircraft fuelsystem 20′. In this example, the aircraft fuel system 20′ includes afuel storage portion 22′ for storing aircraft fuel. The aircraft fuel isconveyed through a fuel passage 24′ to an aircraft engine 26′, such as ascramjet engine. A fuel stabilization unit 27′, such as a fueldeoxygenator, associated with the fuel passage 24 removes oxygen fromthe fuel. In the illustrated example, the aircraft engine 26′ includes acompression section 46, a combustion section 48, and an exhaust section50 that operate in a known manner to propel an aircraft. In thisexample, the fuel passage 24′ conveys the aircraft fuel from the fuelstorage portion 22′ to the combustor section 48.

The fuel passage 24′ includes walls 30′ that define the passageway forthe aircraft fuel. In the illustrated example, a catalyst 32′ adjacentthe walls 30′ endothermically cracks the aircraft fuel to increase thecooling capacity of the fuel for absorbing heat 36′ from the aircraftengine 26′. In one example, the heat 36′ is produced by the compressorsection 46, the combustor section 48, the exhaust section 50, componentsassociated with these sections, or combinations thereof. Additionally,in this example, the fuel passage 24′ is located relatively close to theaircraft engine 26′ such that nascent coke or coking precursors that mayform as a result of the additional absorbed heat may not have time topolymerize before the aircraft fuel is conveyed into the combustionsection 48. Furthermore, even if a limited amount of coking occurs inthe disclosed examples, the catalyst 32, 32′ provides the benefit ofresisting or preventing the coke from adhering to the walls 30, 30′.

The cracking reactions of the disclosed examples, such as cleaving ofcarbon-to-carbon bonds or dehydrogenation, have an associated ΔH ofreaction. For example, table 1 below shows ΔH values for a variety ofexample cleaving and dehydrogenation reactions. Thus, for a givenaircraft fuel having a known composition, one can determine or estimatethe amount of additional heat that the aircraft fuel can absorb (i.e.,cooling capacity) from cracking reactions.

ΔH (77 F.) Reaction (Btu/lb) Dehydrogenation of straight chains 1745C₄H₁₀ → C₄H₆ + 2H₂ Dehydrogenation of cyclics 1050 C₆H₁₂ → C₆H₆ + 3H₂Ring Fracture of Aromatics 3290 c − C₆H₆ → 3C₂H₂ De-dimerization 270dicyclopentadiene → 2c − C₅H₆ Dehydrocyclization 1020 C₇H₁₆ → toluene +4H₂ Dehydrogenation/de-dimerization 1035 JP-10 → 2c − C₅H₆ + 2H₂

In the disclosed examples, the catalyst 32, 32′ includes a transitionmetal selected from zirconium, hafnium, tantalum, niobium, molybdenum,tungsten, rhenium, and combinations thereof. In a further example, oneor more selected ones of the transition metals are used as a transitionmetal compound that includes an carbide, nitride, oxynitride,oxycarbonitride, oxycarbide, phosphide or combination thereof.

In one example, the oxycarbide, oxynitride or oxycarbo-nitride structureis further promoted or tuned by the addition of elements selected fromthe group aluminum, scandium, titanium, vanadium, chromium, manganese,silicon, thorium, and combinations thereof to impart additionalcharacteristics as desired, such as increased activity or to inhibitsintering.

In another example, other elements selected from platinum, palladium,rhodium, iridium, ruthenium, osmium, and combinations thereof are usedin the composition to promote lower temperature “light-off” or sulfurtolerance.

In one example, the transition metal is tantalum, niobium, molybdenum,tungsten or combinations thereof used as a compound (e.g., oxynitride).These example transition metals provide the benefit of catalyticbi-functionality. For example, catalysts 32, 32′ include acid catalyticsites and dehydrogenation catalytic sites. In one example, the acidcatalytic sites include a non-metallic atom, such as oxygen, within ananostructure lattice formed by the catalyst 32, 32′ and thedehydrogenation catalytic sites include a metallic atom within thenanostructure lattice of the catalyst 32, 32′. The acid catalytic sitesfunction in cracking reactions to cleave carbon-to-carbon bonds withinthe aircraft fuel. The dehydrogenation catalytic sites function tocleave hydrogen atoms off of hydrocarbons within the aircraft fuel.Furthermore, the disclosed catalytic materials provide the advantage ofthermodynamic stability over desired periods of time at severelyelevated temperatures, such as between 850° F. and 2000° F. whereconventional types of catalysts become unstable.

In a further example, the catalyst 32, 32′ includes niobium oxynitride(NbN_(x)O_(y)) or niobium oxycarbide (NbC_(x)O_(y)). These are thoughtto be suitable for use with shale oil fuels to produce multiple types ofcracking reactions. It is believed that niobium oxynitride and niobiumcarbide are particularly effective in cracking shale oil fuels throughcarbon-to-carbon cleaving and dehydrogenation reactions for augmentingthe cooling capacity of the aircraft fuel.

The insertion of carbon, nitrogen, or phosphorous into the nanostructurelattice of the transition metal compound increases the lattice parametera_(o) (e.g., for Nb a_(o)=330.6 pm, NbN a_(o)=439.2 pm, and for NbCa_(o)=447.0 pm) to thereby decrease the d band as (a_(o))⁻⁵. Thisincreases the density of d electronic states at the Fermi level.Therefore, the catalytic properties of the Group 4, 5 and 6 transitionmetals shift to the right behaving more like Groups 8, 9 and 10transition metals.

In one example, the catalyst 32, 32′ is selected to augment the coolingcapacity of a given, predetermined type of aircraft fuel. For example,the aircraft fuel may have a composition of non-aromatic hydrocarbonchains. The catalyst 32, 32′ dehydrogenates the hydrocarbon chain toproduce a hydrogen molecule and an adsorbed olefin. The olefin thencracks (i.e., cleaving of one or more carbon-to-carbon bonds) at an acidcatalytic site. If one of the remaining hydrocarbon chains is betweensix and eight carbon atoms in length, it cyclizes and, with theassistance of a hydrogenation/dehydrogenation site, becomes an aromatic(e.g., benzene C₆, toluene C₇, or a xylenes C₈). The cracking of thenon-aromatic hydrocarbon chain results in absorption of heat that isapproximately proportional to the ΔH of reaction for cleaving the typeof non-aromatic hydrocarbon chain. The aircraft fuel absorbs anotheramount of heat that is approximately proportional to the ΔH of reactionfor the dehydrogenation of the aromatic hydrocarbon product. Thus, inthis example, the catalyst 32, 32′ provides the benefit of crackingmultiple constituents within the aircraft fuel to augment the amount ofheat that the aircraft fuel can absorb and thereby increase the coolingcapacity of the aircraft fuel.

Optionally, the catalyst 32, 32′ includes a first catalyst section 60 aand a second catalyst section 60 b (FIGS. 1 and 2). The first catalystsection 60 a is located upstream from the second catalyst section 60 b.In this example, the catalyst 32, 32′ of the first catalyst section 60 aincludes a first transition metal and the catalyst 32, 32′ of the secondcatalyst section 60 b includes a different, second transition metal(e.g., from those mentioned above). In one example, one or both of thecatalysts 32, 32′ of the first catalyst section 60 a and the secondcatalyst section 60 b are transition metal carbides, nitrides,oxynitrides, oxycarbonitrides, oxycarbides, phosphides or combinationsthereof. The different transition metals selectively crack differenttypes of compositional constituents or are particularly suited forcertain types of cracking reactions (e.g., cleaving carbon-to-carbonbonds, dehydrogenation, etc.). This provides the benefit of tailoringthe activity of the catalyst 32, 32′ along the length of the fuelpassage 24. For example, the first catalyst section 60 a cracks theaircraft fuel into a constituent product and the second catalyst section60 b cracks the constituent product downstream to augment the amount ofheat absorbed by the aircraft fuel.

In one example, the first catalyst section 60 a is located in arelatively lower temperature area than the second catalyst section 60 b.The area may be separated or adjacent as shown. In this example, thecatalyst 32, 32′ of the first catalyst section 60 a includes a catalystmaterial suitable for operation at the relatively lower temperature. Forexample, the catalyst 32, 32′ of the first catalyst section 60 aincludes a zeolite type catalyst having an aluminum oxide or,alumino-silicate material. This provides the benefit of tailoring theactivity of the catalyst 32, 32′ along the length of the fuel passage24, depending on the expected temperatures in the locations of thecatalysts 32, 32′. Alternatively, more than two catalyst sections areused as taught above to crack a plurality of constituent products over aplurality of sections having differing temperature to tailor theactivity of the catalyst 32, 32′ along the fuel passage 24. Given thisdescription, one of ordinary skill in the art will recognize alternativearrangements to suit their particular needs.

Alternatively, or in addition to having different transition metals inthe first catalyst section 60 a and the second catalyst section 60 b, aratio of acid catalytic sites to dehydrogenation catalytic sites isselected to augment the cooling capacity. In one example, the ratio inthe first catalytic section 60 a differs from the ratio in the secondcatalytic section 60 b. Depending upon the type of aircraft fuel, ahigher ratio of acid catalytic sites may be desired within the firstcatalyst section 60 a to promote more cleaving of the carbon-to-carbonbonds. Downstream in the second catalyst section 60 b, a greater numberof dehydrogenation catalyst sites may be desired to dehydrogenize theconstituent products resulting from the reaction at the first catalystsection 60 a. Thus, the types of transition metals used, the ratio ofacid catalytic sites to hydrogenation catalytic sites, or both, may bevaried along the length of the catalyst 32, 32′ within the fuel passage24 to achieve a desired amount of heat absorption.

In one example, the first catalyst section 60 a includes either niobiumoxynitride or niobium oxycarbide, as described above, and the secondcatalyst section 60 b includes molybdenum oxynitride (MoN_(x)O_(y)). Inthis example, the niobium oxynitride or niobium oxycarbide has a greaterratio of acid catalytic sites than the molybdenum oxynitride, and themolybdenum oxynitride has a greater ratio of dehydrogenation catalyticsites than the niobium oxides.

Furthermore, the types of transition metals selected and the ratio ofcatalytic sites may be selected based upon the type of fuel expected tobe used. For example, certain transition metals or combinations thereofmay be most suitable for one or more type of fuel such as petroleumfuels (e.g., paraphenic fuel, naphthenic fuel) having relatively largeconcentrations of ringed structures, Fischer-Tropes fuel havingrelatively large concentration of non-aromatic, hydrocarbon chains,certain shale oil fuels having relatively high concentration of ringedhydrocarbons, or bio-fuels having relatively high concentrations ofnon-aromatic hydrocarbon chains with unsaturation (e.g., C₁₆ fatty acidesters).

Additionally, the expected environmental temperatures may be taken intoaccount when selecting the catalyst 32, 32′. For example, catalystmaterials having relatively higher melting temperatures may be bettersuited for higher temperature environments. To list a few examples, themelting temperatures of molybdenum nitride, molybdenum carbide, tungstencarbide, and tungsten nitride are 1750° C., 2692° C., 2780° C., andabout 2000° C., respectively. Given this description, one of ordinaryskill in the art will be able to determine which types of catalysts aresuitable for their particular needs.

In the disclosed examples, the above-mentioned catalysts 32, 32′ alsoinclude a desirable porosity to provide the benefit of operationalstability and resistance to coking. In one example, the porosity of thecatalysts 32, 32′ is tailored to minimize mass transfer resistancetypified by 1.5-10 nm pores while achieving a favorable cokesolubilization in <1 nm pores.

In some catalyst systems, such as zeolite systems, it has been observedby in situ Cylindrical Internal Reflectance FTIR (CIR-FTIR) that thedensity of the hydrocarbon fuel within the zeolitic micropores wasliquid-like and at least one order of magnitude higher than that thatpredicted by the thermodynamics (equation of state) under supercriticalstate. This is attributed to a “condensation-type” effect that isfurther enhanced by the supercritical state, which alters the nature ofcarbon-hydrogen chemical bonds in the molecules (broadening in thespectrum peaks for the carbon-hydrogen stretching bonds). Stabilizationof the catalyst against deactivation by coke formation was observed forthese supercritical processing conditions, resulting in a practicallystable catalyst for long operation times.

The hydrocarbon “condensation-type” effect in the catalyst microporesfor a given set of supercritical processing conditions is a function ofprimarily two parameters: 1) the diameter of the catalyst micropore and2) the local electric field in the catalyst micropore as defined in thecase of the zeolites by the Si/Al ratio and the cation electric chargeand effective diameter. Thus, the stabilization of a catalyst againstdeactivation by coke formation can be controlled, at least in part, byutilizing ≤1.5 nm diameter pores that promote the hydrocarbon“condensation-type” effect to solubilize coke and coke precursors. Inone example, pores of the catalysts 32, 32′ are interconnected in a porenetwork with up to about 20% of the surface area in pores ≤1.5 nm indiameter, about 20-30% of the surface area in pores between about 1.5 nmand 3 nm, about 40% or more of the surface area in pores between 3 nmand 6 nm with the balance in pores greater than 6 nm in diameter (masstransfer resistance depends on the pore diameter to the −1.5 power). Anexcess of large pores (those >6 nm in diameter) will tend to decreasethe density of the supercritical fuel toward the predicted thermodynamicvalue (i.e., for this example that is gaseous). That is, the coke andcoke precursor solubilization efficiency is limited by an excess oflarge pores and the catalysts 32, 32′ will lose the initially higheractivity rather rapidly. Given this description, one of ordinary skillin the art will recognize other desirable porosities suited for theirparticular needs.

A desirable porosity (e.g., fractal pore structure) may be used incombination with an appropriately balanced number and type of activesites on the walls of these pores (i.e., and appropriate ratio of acidand hydrogenation/dehydrogenation sites). This balance furthermore is afunction of the fuel type and the operating temperature.

In the disclosed examples, the catalyst 32, 32′ generally decreases incatalytic activity as the catalytic sites are used up in the crackingreactions. In one example, a carbon atom replaces the atom of thecatalytic site such that the catalytic site has reduced catalyticactivity. Over a period of time, the overall catalytic activity of thecatalyst 32, 32′ is reduced in this manner. In one example, a controlledamount water is added to the aircraft fuel to regenerate the catalyticsites. Under high pressure within the fuel passage 24, the waterprovides a source of oxygen for regenerating the catalytic sites. Forexample, an oxygen atom replaces the carbon atom at the catalytic siteto increase the catalytic activity of the catalytic site. Optionally,the water is injected under pressure into the aircraft fuel toselectively regenerate the catalyst 32, 32′ at a desired time, such asat an interval over a long flight. Alternatively, other regeneratingfluids are used in addition to or instead of water. In one example, theregenerating fluid includes an ammonia/water mixture, acetonitrile,methanol, dimethyl ether, ethanol, amino alcohol, or other oxygenate.The addition of nitrogen compounds like ammonia is counter intuitive asnitrogen bases typically poison active sites, however in this instancethe acid sites depend in part of the presence oxynitrides that may bereacted away and replaced with carbides unless restored through thejudicious addition of nitrogen and oxygen in the proper form.

In the disclosed examples, the catalyst 32, 32′ is disposed on or nearthe walls 30, 30′ of the fuel passage 24 in any of a variety of knownmethods. In one example, the walls 30, 30′ are pretreated with oxygenand steam to oxidize the surfaces thereof. This provides the benefit ofproducing an oxide layer on the walls for better adhesion with thecatalyst 32, 32′.

The pretreated walls 30, 30′ are then wash-coated with a suspension ofnano-sized catalytic precursor particles, such as transition metaloxides. The particles, or individual crystallites or proto-crystallitesor the oxide or proto oxide, are approximately less than 5 nm in sizeand forms porous particles on the order of 50 to 200 nm in diameter. Inone example, the washcoat includes pores that are approximately 2 nm orlarger in diameter and a surface area of greater than about 800 m²/g percm³ of skeletal volume. For a material with a skeletal density of 4g/cm³ the 800 m²/cm³ skeletal volume translates to 200 m²/g.

The washcoat is then heated under reducing conditions with hydrogen andammonia, a light hydrocarbon gas, or oxygenated hydrocarbon depending onwhether the desired catalyst 32, 32′ is an oxynitride or oxycarbide.Optionally, the washcoat is also exposed to low partial pressures ofoxygen or an oxygen containing compound after reduction to adjust theoxygen level before a passivation step. During the passivation step, thewashcoat is cooled in inert gas to room temperature and with a graduallyincreasing oxygen environment. In one example, the oxygen concentrationis increased up to ambient (˜20% oxygen). To insure rapid light off whenin use the resultant oxynitride or oxycarbide catalyst 32, 32′ is loadedwith one or more noble metals from the group consisting of Re, Ru, Rh,Pd, Os, Ir, Pt, Au. The above method is one example of forming thecatalyst 32, 32′ having a lattice network structure containing atransition metal skeleton with oxygen atoms scattered throughout thelattice. Given this description, one of ordinary skill in the art willrecognize alternative processing methods for depositing the catalyst 32,32′ on the walls 30 of the fuel passage 24 in desired thicknesses andpore size distributions.

Although a preferred embodiment of this invention has been disclosed, aworker of ordinary skill in this art would recognize that certainmodifications would come within the scope of this invention. For thatreason, the following claims should be studied to determine the truescope and content of this invention.

We claim:
 1. A method of controlling cooling in an aircraft system,comprising: endothermically cracking a fuel to increase cooling capacityof the fuel using a catalyst that includes at least one transition metalcompound of at least one of carbides, nitrides, oxynitrides,oxycarbonitrides, oxycarbides, phosphides, and combinations thereof andthe at least one transition metal includes at least one of zirconium,hafnium, tantalum, niobium, molybdenum, tungsten, platinum, palladium,rhodium, iridium, ruthenium, osmium, rhenium, and combinations thereof;and using the cracked fuel to cool a heat source comprising an aircraftcomponent.
 2. The method as recited in claim 1, wherein the endothermiccracking includes cleaving carbon-to-carbon bonds of the fuel.
 3. Themethod as recited in claim 2, wherein the carbon-to-carbon bonds are ofnon-aromatic hydrocarbon chains of the fuel.
 4. The method as recited inclaim 1, wherein the endothermic cracking includes dehydrogenation ofthe fuel.
 5. The method as recited in claim 1, wherein the endothermiccracking includes endothermically cracking a pre-selected type of acompositional constituent of the fuel to absorb a first amount of heatand to produce a product compositional constituent.
 6. The method asrecited in claim 5, wherein the endothermic cracking includesendothermically cracking the product compositional constituent to absorba second amount of heat.
 7. The method as recited in claim 6, whereinthe at least one transition metal includes a first transition metal anda second, different transition metal, and the endothermic crackingincludes endothermically cracking the predetermined type of thecompositional constituent using a the first transition metal andendothermically cracking the product compositional constituent using thesecond, different transition metal.
 8. The method as recited in claim 1,further comprising regenerating the catalyst to increase catalyticactivity.
 9. The method as recited in claim 8, wherein the regeneratingincludes adding an oxygenate or a nitrogen-containing compound to thefuel as a source of, respectively, oxygen or nitrogen for regeneratingthe catalyst.
 10. The method as recited in claim 1, wherein the at leastone transition metal compound further includes at least one of aluminum,scandium, titanium, vanadium, chromium, manganese, silicon, thorium, andcombinations thereof.
 11. The method as recited in claim 1, wherein theat least one transition metal is selected from the group consisting oftantalum, niobium, molybdenum, tungsten, and combinations thereof. 12.The method as recited in claim 1, wherein the catalyst includes at leastone of niobium oxynitride, niobium oxycarbide, and molybdenumoxynitride.
 13. The method as recited in claim 1, wherein the at leastone transition metal compound includes molybdenum oxynitride and atleast one of niobium oxynitride or niobium oxycarbide, and theendothermic cracking includes endothermically cracking a predeterminedtype of a compositional constituent using the at least one of niobiumoxynitride or niobium oxycarbide to produce a product compositionalconstituent, and endothermically cracking the product compositionalconstituent using the molybdenum oxynitride.